A plasma gun or torch is a device used to apply spray coatings at high temperatures and velocities to a surface. A conventional plasma gun is comprised of generally tubular channel with a cathode assembly at one end and an anode assembly at the other. When a sufficiently high voltage is applied across the anode and cathode, an electric arc is generated. Gas is fed into the chamber at one end and is heated by the arc to form a plasma. An exit nozzle is provided at the other end of the chamber to direct the plasma. The powder to be sprayed is injected into the plasma stream. The powder is heated and accelerated by the plasma and can be sprayed onto a surface to be coated. By controlling the voltage and the rate of gas flow, the amount of heating and velocity of the generated plasma, and thus the temperature and spray velocity of the powder, can be adjusted.
A general objective for plasma spray guns is to provide uniform heating and acceleration for as much of the injected powder as possible. When powder particles experience same heating and acceleration conditions, the resulting coating is more uniform. As variations are introduced into the temperature and velocity of the powder, defects in the coating can result, reducing the overall effectiveness of the coating. In addition, by providing uniform heating and acceleration, the efficiency at which powders are deposited is increased.
The plasma arc will generally attach to various points on the anode, where the specific attachment point depends on the lowest energy path between the cathode and the anode. In order to reduce erosion of the anode by the plasma arc, many spray guns use multiple arcs. For example, the plasma gun design disclosed in U.S. Pat. No. 5,406,046 uses three cathodes to produce three arcs which attach to different fixed points on a circular anode. Compared to a single arc, the current flow in each of the three arcs is reduced to one-third and the erosion of anode is reduced to one-ninth.
With reference to FIGS. 1A and 1B, the output of a three arc plasma gun 10 is a three plume plasma structure 12. In a one-plume structure, the powder injection velocity must be controlled to place the powder in the center of plume without under or overshooting. In a three plume structure, as shown in FIGS. 1A and 1B, the powder injectors 14 are preferentially arranged so that powder 16 is injected directly between two adjacent plumes 12a, 12b and towards the third 12c, as shown in FIG. 1B. This produces a caging effect that directs the powder into the desired central area between the three plumes.
The gas passing through the gun is typically swirled, as shown in FIG. 1A, to permit more uniform heating of the gas by the arcs. The electrical arcs generally follow the path of heated gas and, therefore, the swirling gas changes the position of the arc attachment points on the anode and, consequentially, the position of the plasma plumes. The amount of change is dependent on the mass and velocity of the gas within the chamber as well as variations in current flow. Because these parameters can vary, and indeed are typically selectable by a user, the location of the plumes does not have a fixed relationship to the body of the gun. Instead, the arcs and corresponding plumes establish themselves in a stable zone according to the instantaneous operating conditions, such as the gas mass flow and the amperage of the current flow.
As will be appreciated, as the position of the plasma plumes change, the optimal injection points also change. With reference to FIG. 1B, a conventional solution is to provide a plasma gun 10 that has powder injector assemblies 12 with radial positions that are adjustable relative to the position of the plasma plumes. For example, the powder injectors can be mounted onto a rotatable injection ring 18. The ring 18 can then be adjusted each time the operating conditions of the gun change to place the powder injectors in the optimal position.
One drawback with this solution is that the positioning not always accurate. In addition to human error and mechanical imprecision, there are also random fluctuations in power and/or input gas flow that will cause wandering of the arc attachment points and subsequent misalignment of the injectors. Because the misalignment affects the temperature and velocity of the applied powder, the changes can result in inconsistent coatings being applied as the position of the injection relative to the plasma plumes varies. In addition, the deposit efficiency can be also be reduced. Since the powder is typically the most expensive component of the coating process, even small changes in deposit efficiency can have non-trivial economic impact.
It is an object of the present invention to provide an improved plasma generator with an anode element that provides arc attachment points that remain radially fixed even as the operating conditions of the gun change.
It is a further object of the invention to provide a plasma generator with an anode element wherein the arc attachment points can vary along respective generally longitudinal axes.